Humidity sensors are useful in many applications such as weather stations, air conditioning, home and office appliances, and industrial instruments. For electronic-type relative humidity sensors, polymer-based humidity sensors dominate the market, due to their low cost and mature manufacturing processes. However, for high performance humidity sensors semiconductor material-based and ceramic material-based sensors are often used.
An anodic porous aluminum oxide (AAO) film (Al foil anodized in an acid solution) is a known ceramic material for the fabrication of humidity sensors. The first humidity-sensitive anodized alumina layer on aluminum substrate was reported in 1953. The capacitance of such a structure was found to increase with relative humidity. Aluminum oxide-based humidity-sensitive materials attracted increased attention since 1978, when a regular honeycomb structure aluminum oxide material was developed. There have been intensive studies to develop high performance and reliable sensors based on such a novel nano-structured material.
Anodizing is a well-developed technique for producing protective AAO layers, for example, on aluminum for structure applications such as window frames. However, anodization techniques for protective applications were not directly applicable to creating AAO-based humidity sensors. The humidity response of an AAO thin film is highly dependent on the anodizing parameters. For example, prior humidity sensors have a capacitance versus humidity response that is often nonlinear, with a ‘flat’ response in low or high humidity; such sensors cannot operate in a full humidity range. The long-term stability of a humidity sensor is another major concern; prior AAO sensors degrade when exposed to high humidity. Proposals to resolve the problems of prior art humidity sensors depended on sophisticated structural designs and/or fabrication processes, resulting in unacceptable manufacturing costs for commercial production. Therefore, only limited progress has been made in forming absolute humidity sensors based on anodic aluminum oxide.
Thus there is a need in the art for cost-effective rapid response humidity sensors bases on anodic aluminum oxide which have a substantially linear capacitance versus humidity response and can measure a full range of humidity conditions.